Image scanners, also known as document scanners, convert a visible image on a document or photograph, or an image in a transparent medium, into an electronic form suitable for copying, storing or processing by a computer. An image scanner may be a separate device, or an image scanner may be a part of a copier, part of a facsimile machine, or part of a multipurpose device. Reflective image scanners typically have a controlled source of light, and light is reflected off the surface of a document, through an optics system, and onto an array of photosensitive devices. Transparency image scanners pass light through a transparent image, for example a photographic positive slide, through an optics system, and then onto an array of photosensitive devices. The optics system focuses at least one line, called a scanline, on the image being scanned, onto the array of photosensitive devices. The photosensitive devices convert received light intensity into an electronic signal. An analog-to-digital converter converts the electronic signal into computer readable binary numbers, with each binary number representing an intensity value.
In some configurations, the light source is a long tube providing a narrow band of light which extends beyond each edge of the document for one dimension. For electric discharge lamps, such as cold-cathode fluorescent lamps, intensity and color is a function of power and temperature. The temperature of the vapor or gas, and the phosphors, indirectly affects intensity. Because of thermal time constants in the lamp, when such a lamp is first powered on, light intensity and color vary dynamically along the length of the tube until the overall temperature of the light source stabilizes.
The time required for complete stabilization may be on the order of many minutes. Image scanners using such a light source typically wait for some stabilization before scanning the document, typically for at least tens of seconds. In general, such a delay adds additional time to every scan. Computer input/output speeds have improved, and scanner performance has improved, to the extent that scanning time and computer input/output time may be less than lamp warm-up time. As scanning times have decreased, decreasing the delay due to lamp warm-up is becoming particularly important.
Lamp warm-up is important for color accuracy, in addition to intensity. The human eye contains three different kinds of color receptors (cones) that are sensitive to spectral bands that correspond roughly to red, green, and blue light. Specific sensitivities vary from person to person, but the average response for each receptor has been quantified and is known as the “CIE standard observer.” Accurate reproduction of color requires a light source that has adequate intensity in each of the spectral response ranges of the three types of receptors in the human eye. Typically, given a set of numerical values for photosensor responses for one pixel, for example, red, green, and blue, the numbers are mathematically treated as a vector. The vector is multiplied by a color transformation matrix to generate a different set of numbers. In general, the coefficients in the color transformation matrix compensate for differences between the response of photosensors and the response of the CIE standard observer, and the coefficients in the matrix may include compensation for the spectrum of the light source. See, for example, U.S. Pat. No. 5,793,884, and U.S. Pat. No. 5,753,906. An example output of the matrix is a set of coordinates in the CIE L*A*B* color space. Typically, matrix coefficients are fixed, and are obtained in a one-time factory calibration using a stable light source. With fixed matrix values, it is typically assumed that the spectrum of the light source is constant along the length of the lamp, and constant during the scan. Accordingly, it is common to wait for the lamp to stabilize before scanning to ensure that the spectrum of the illumination is close to the spectrum assumed in the matrix values.
There have been many approaches to accommodating lamp warm-up time or decreasing lamp warm-up time. Image scanners may simply wait open-loop for a worst case lamp warm-up time before initiating a scan. As one alternative to open-loop waiting, some image scanners leave the lamp on continuously. Fluorescent lamps for image scanners are relatively low power, so that continuous usage does not waste much power, but consumers may be concerned about the apparent waste of power and possible reduced lifetime.
In some scanners, the lamp is kept warm without being powered on continuously. For example, in some image scanners, the lamp is periodically turned on for a few minutes every hour during long periods of inactivity (see U.S. Pat. No. 5,153,745). In some scanners, the lamp is enclosed by a heating blanket (except for an aperture for light emission), which keeps the lamp continuously warm (see U.S. Pat. No. 5,029,311).
As another alternative, some image scanners overdrive the lamp initially to decrease the warm-up time (see U.S. Pat. No. 5,907,742; see also U.S. Pat. No. 5,914,871). In '742, the lamp current is also maintained at a low level between scans to keep the lamp warm.
Still another approach is to monitor a lamp parameter during warm-up, and delay scanning until the parameter is stable. For example, see U.S. Pat. No. 5,336,976, in which power to the lamp is monitored, and scanning is delayed until power stabilizes.
Even with a warm lamp, intensity varies along the length of the lamp. In particular, for a warm lamp, the center region of the lamp is typically brighter than the ends of the lamp. Reflective document scanners and copiers commonly have a transparent platen on which a document is placed for scanning. Reflective document scanners and copiers commonly provide a fixed-position calibration target (also called a calibration strip), along a scanline dimension, typically along one edge of the bottom surface of the platen. This calibration target is used to compensate for variation in sensitivity of individual photosensors (photo-response non-uniformity or PRNU), and for variation in light intensity along the length of the scanline. See, for example, U.S. Pat. No. 5,285,293.
PRNU is a measure of the output of each photosensor compared to the expected voltage for a particular target calibration target and light source. The calibration process compensates for at least four different factors: (1) non-uniform photosensor sensitivity, (2) non-uniform illumination, (3) cosine-fourth falloff of light at an angle relative to the optical axis of a lens, and (4) contamination in the optical path (for example, dust on a lens or other optical components). The first, third, and fourth factors are typically constant during a scan. The second factor may vary during a scan if lamp temperature has not stabilized. The primary concern of the present patent document is the variable second factor, but the PRNU calibration and compensation process includes calibration and compensation for the other factors as well.
FIG. 1 (prior art) illustrates an example of a system for performing PRNU compensation during scanning. FIG. 1 is not intended to literally represent any particular system, but instead is intended to illustrate the compensation functions being performed. In FIG. 1, a photosensor array 100 transfers charges to a charge shift register 102. Charges are serially shifted from the charge shift register 102 and converted to voltages. The resulting voltages pass through a summing junction 104 to an amplifier 106. A processor 110 has associated memory 108. Outputs from the amplifier 106 are converted by an analog-to-digital (A/D) converter 116 for reading by the processor 110. Digital outputs from the processor 110 are converted by digital-to-analog (D/A) converters 112 and 114. Before scanning, outputs from the photosensors 100 are measured, without exposure to light, to measure thermal noise (also called dark noise). The resulting digital dark noise values are stored in the memory 108. Also before scanning, the photosensors 100 are exposed to light from a calibration target, and the resulting digital values are used to compute amplifier gain values that are stored in the memory 108. Essentially, the amplifier gain values ensure that, after compensation, the outputs of the amplifier are identical for all photosensors when viewing the calibration target. Then, during scanning, stored dark noise values are converted to voltages by D/A converter 112, and the resulting voltages are subtracted from corresponding image voltages at the summing junction 104. Stored amplifier gain values are converted to voltages by D/A converter 114, and the resulting voltages are used to control the gain of amplifier 106. The resulting image voltages, with noise offset and gain compensation, are converted by A/D converter 116 and are typically then sent to a host computer, or to some other destination for storing, printing, or transmitting.
If PRNU calibration is made while the intensity of the light source is still dynamically changing, an inaccurate sensor calibration may result. As a result, even though the intensity of the light source may be stable for most of the scan, the sensors will be inaccurate for the entire scan because of inaccurate initial calibration. Accordingly, it is common to wait for the lamp to stabilize before doing the PRNU calibration.
Even after the lamp is warm, there may be some intensity variation over time. Reflective document scanners and copiers also commonly provide a second calibration target along one edge of the platen in the direction of scanning travel. This second calibration target is used to compensate for variation in lamp intensity during a scan. Essentially, it is assumed that once the lamp is warm, then relative intensity variation along the length of the lamp is constant, so it is sufficient to measure intensity near one end of the lamp. See, for example, U.S. Pat. No. 5,278,674. It is also known to monitor the color of the lamp (again, just near one end), for gain compensation. For scanners having a moving carriage, with the lamp in the moving carriage, it is also known to provide a small tab on the moving carriage for intensity monitoring at one end of the lamp. See U.S. Pat. No. 6,028,681. Similarly, for a hand held scanner, it is known to provide small intensity calibration areas within the scanner, near the ends of the light source, and the entire scanner moves relative to a document being scanned. See U.S. Pat. No. 5,995,243.
In U.S. patent application Ser. No. 09/772,714, a scanner has a calibration target that is visible to a photosensor array continuously during a scan. For example, if the lamp is in a moving carriage, the calibration target may be on the moving carriage. At least one separate array of photosensors is used to continuously monitor the intensity of the illumination, along the calibration target, during a scan.
In U.S. patent application Ser. No. 09/842,306, a scanner performs an initial calibration for lamp intensity before scanning, and a final calibration for lamp intensity after scanning. At least some compensation is performed after scanning is completed, using calibration values computed by interpolating between the initial calibration values and the final calibration values.
In U.S. patent application Ser. No. 09/842,306, a separate array of photosensors directly monitors the lamp during scanning.
There is an ongoing need to reduce the delay associated with lamp warm-up, and to provide PRNU calibration, intensity compensation, and color compensation, during scanning.